[jsonrpc] Always return empty TV shows via VideoLibrary.GetTVShows.

Ensure that TV shows without any episode are returned when calling
VideoLibrary.GetTVShows even if videolibrary.hideemptyseries is true in
advancedsettings.xml.

The JSON-RPC API version is incremented so that clients know that results
before 6.20.2 are unreliable.